1. Field of The Invention
The subject invention relates to a ghost cancellation system and a circuit for detecting and synchronizing to a ghost cancellation reference signal contained in a television signal.
2. Description of The Related Art
U.S. Pat. Nos. 5,047,859 and 5,121,211 both to Koo disclose a signal which, when transmitted with a standard television signal, enables a corresponding circuit to correct said signal thereby eliminating "ghost", i.e., errors in the signal arising from, for example, reflections from buildings, terrain, etc. The ghost cancellation reference (GCR) signal provided in the television input video signal vertical interval is used as an input to an adaptive filter algorithm. This algorithm provides coefficients to digital filters which cancel secondary echo-like signals from the main input signal. For a ghost canceler to be effective, the ghost cancellation reference (GCR) signal must be sampled by an analog-to-digital (A/D) converter and the sampled signal compared to a reference signal. The GCR signal is transmitted every field and is sequenced so that all other static video characteristics can be discriminated and removed algebraically over an 8 field period. For optimum performance, the sampling clock should maintain phase coherence from one field sample set to the next. There should also be a zero phase difference between the sampled GCR signal and a stored reference GCR signal.
Presently, video characteristics are used to maintain phase coherence between sample sets. This is accomplished by phase-locked loop circuits using either the chroma burst or the horizontal synchronization signal as a reference. If the reference phase is sufficiently detected, then the phase coherence between sample sets can be maintained. However, badly ghosted signals severely distort both the chroma burst and the horizontal synchronization signal. As such, the phase-locked loop circuits cannot provide phase coherence and, in addition, provide additional phase errors.
It is unlikely that all phase-locked loop circuits can provide a zero phase difference between the sampled GCR signal and the stored reference GCR signal because the steady-state error of all circuits would have to be equal and sufficient to provide the correct phase. Additionally, the GCR signal has a flat spectrum in the range 0 to 4.2 MHz. over several sampling clocks. A phase-locked loop circuit which is pulling the sampling clock over the sample set will distort the spectrum as well. Phase-locked loop circuits using the video horizontal synchronization signal have a bandwidth sufficiently wide to allow frequency incoherence between sample sets. This is especially true under ghosting conditions because the reference synchronization signals are highly prone to error.